The primary parameters for characterizing the performance of an optical amplifier are the gain, the output power and the noise figure. The gain and output power of an amplifier are relatively easy to measure. However, the accurate determination of the noise figure is more difficult, particularly in the case of saturated operation, which is of great practical importance. The problem arises when the amplified spontaneous emission (ASE) spectral density is measured at the signal wavelength in the presence of a large input signal. A common technique for measuring the noise figure of, for example, an erbium doped fiber amplifier involves fitting a curve to the ASE level near the signal and then extrapolating the curve to find the ASE noise level at the signal wavelength. The major problem with this technique is that the ASE spectrum is distorted by the sidebands of the signal source and by the optical spectrum analyzer response in the presence of a strong signal.
It is an object of this invention to provide a method and apparatus for more accurately measuring the noise figure of an optical amplifier under various operating conditions.